The present invention relates to an integrated circuit composed of a semiconductor layer sequence that is grown on a semi-insulating substrate and includes at least one Schottky diode as well as at least one heterostructure field effect transistor (HFET), and to a method of producing such an integrated circuit.
The invention is used in the millimeter wave communications art and in the sensor art as well as in radar, traffic control and satellites.
In communications and sensor systems, a further raise of the operating frequencies into the millimeter wavelength range brings about improved beam characteristics and increased resolution. Of particular interest in this connection are frequencies in the range of the atmospheric windows at 94 GHz, 140 GHz and 220 GHz for communications systems and the absorptions bands created by oxygen and water in the frequency ranges therebetween for short-range systems. However, this requires integrated circuits that have the appropriate gain, noise and mixer characteristics in the millimeter wavelength range, the so-called MMIC's (monolithically integrated millimeter wavelength circuits). The individual components must therefore always have higher cutoff frequencies and a suitable high frequency behavior.
It is known to manufacture electronic components for high operating frequencies (ultra-high frequency components) from semiconductor starting materials which were produced by epitaxial growth methods, e.g., a MBE process or a CVD process. The epitaxy causes different layers that determine the function of the component to be precipitated on a substrate. These layers are structured by means of lithographic processes (photo- and electron beam lithography) and etching processes (wet and dry chemical processes).
It is also known for a system of GaAs materials to produce semi-insulating regions in previously conductive layers by bombarding them with boron ions or protons, and to produce n-conductive regions in previously semi-insulating regions by bombarding them with silicon ions and subsequent short-term healing during the production of semiconductors.
Developments in recent years have shown that the thus far highest amplifier cutoff frequencies can be realized with heterostructure field effect transistors (HFET's). For uses in the millimeter wavelength range, the HFET is superior to conventional components, particularly the MESFET component, primarily in its noise behavior and in its high frequency amplifying characteristics. Moreover, due to its physical mode of operation, an HFET is usable for cryogenic applications down to very low temperatures.
For operating frequencies far into the sub-millimeter wavelength range, GaAs Schottky diodes have at present the best mixer characteristics for room temperature applications (see D. G. Garfield, R. J. Mattauch and S. Weinreb, "RF Performance of a Novel Planar Millimeter-Wave Diode Incorporating an Etched Surface Channel," in IEEE Transactions on Microwave Theory and Techniques 39 (1), 1991, pages 1-5, and the references cited therein).
With a monolithically integrated structure of an electronic system, parasitic losses, particularly at the interfaces between the various components can be minimized. In addition, installation costs can be noticeably lowered due to the small amount of hybrid configuration technology employed.
One method of producing planar MMIC's of GaAs MESFET's and Schottky diodes for operating frequencies below 100 GHz is disclosed by A. Colquhoun, G. Ebert, J. Selders, B. Adelseck, J. M. Dieudonne, K. E. Schmegner, and W. Schwab, "A Fully Monolithic Integrated 60 GHz Receiver," in Proceedings of the Gallium Arsenide IC Symposium, 1989, San Diego, Calif., pages 185-188.
A formulation for the integration of HFET's with Schottky diodes in a non-planar geometry was published by W. J. Ho, E. A. Sovero, D. S. Deadin, R. D. Stein, G. J. Sullivan, and J. A. Higgens, in an article entitled "Monolithic Integration of HEMT's and Schottky Diodes for Millimeter Wave Circuits," Rec. of the IEEE GaAs Integrated Circuits Symposium, 1988, pages 139-242. This involves epitaxially stacking an n.sup.+ nn.sup.+ GaAs diode layer sequence on an AlGaAs/GaAs layer sequence for the HFET. A prerequisite for the realization of the highest diode cutoff frequencies, however, is a very low parasitic resistance and a low capacitance which can be realized only with very thick n.sup.+ lead layers (typically at least in a range around 1 .mu.m) and an active layer having a thickness between 0.5 .mu.m and 1 .mu.m (see D. G. Garfield et al, IEEE Transactions on Microwave Theory and Techniques 39 (1), 1991, pages 1-5). Thus there is an unavoidably great difference in height between the diode and the HFET in the proposed structure. The non-planarity makes the engineering of the process of manufacturing the circuit considerably more difficult.